In recent years, research and development of a high density optical disc system capable of recording and/or reproducing information by using a violet semiconductor laser with a wavelength of about 400 nm, are advancing rapidly. As an example, in the case of an optical disc on which information is recorded and/or reproduced under the specifications that NA is 0.85 and a light source wavelength is 405 nm, namely, in the case of the so-called Blu-ray Disc (hereinafter referred to as BD), it is possible to record information of about 25 GB per layer for an optical disc having a diameter of 12 cm.
In an optical pickup device that can record and/or reproduce information on the BD, an objective lens is made with glass or an objective lens is made with plastics. An objective lens made with glass is not suitable for mass production and it tends to be high in cost, though it can exhibit stable optical characteristics in general. In contrast to this, an objective lens made with plastics has its merits that it is suitable for mass production through injection molding, and it makes cost reduction to be possible in general, with only one problem that temperature characteristics are relatively poor. The temperature characteristics mean a level of an amount of aberration generated in a light flux passing through an objective lens according to the ambient temperature change. The temperature characteristics can be improved by adding a ring-shaped structure like a diffractive structure on an optical surface, but wavelength characteristics are worsened in many cases, instead. The wavelength characteristics mean a level of an amount of aberration generated in a light flux passing through an objective lens according to a wavelength change of a light flux emitted from a light source.
For the background mentioned above, Patent Literature 1 discloses an objective lens made with plastics with NA of 0.85 wherein an amount of correcting the temperature characteristics by a ring-shaped structure is reduced by shortening a focal length, and thereby, both of the temperature characteristics and the wavelength characteristics are improved.
Patent Literature 1: JP-A No. 2004-252135